Gas separation membranes have been in industrial use for close to 25 years. Various types of membrane are available, although almost all commercially successful membranes are polymeric membranes formed as flat sheets or hollow fibers.
For use, it is desirable to pack a large membrane area into a small volume. Because membranes are delicate, susceptible to damage and may have a limited operating life, they are normally factory-built into modules or elements.
Two predominant types of membrane modules have emerged. If the membranes are in hollow-fiber form, bundles or hanks of closely spaced fibers are potted in a cylindrical pressure housing or tube. A single hollow fiber module may contain as much as 1,000 km of fiber. Feed gas may flow on the shell or bore side of the fibers. The permeate gas may be routed to a single collection pipe by which it exits the module.
Flat-sheet membranes are commonly packaged into spiral-wound modules. A spiral-wound module has a central perforated permeate collection tube, around which are wound membrane envelopes interleaved with spacers to define feed and permeate channels. The module is usually finished with an outer wrap of fiberglass or the like and mounted in a pressure housing. Feed passes axially down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals towards the center and exits through the collection tube, which may protrude at one or both ends beyond the membrane envelope. Modules typically contain multiple membrane envelopes, such as 20 or more, and have a usable membrane area of up to 40 or 50 m2.
Less commonly, membrane elements may take other forms. For example, they may simply be rigid tubes, such as ceramic tubes, with the selective membrane as the inside or outside surface.
Modules or elements are normally built in standard sizes. Ideally, the standard size module is limited to that which can be manhandled easily by one or two persons, with a weight up to about fifty pounds. The requirement for a specific membrane area is met by installing the appropriate number of factory-made elements in suitable permanent housings. If the membranes need to be replaced in the field, the complete module is removed from the housing and replaced with a new factory-built module. This procedure minimizes any plant downtime required for membrane replacement.
In gas processing plants, gas flows and membrane performance are such that membrane area requirements may be very large, in the range of hundreds, thousands or even tens of thousands of square meters. Large numbers of modules need to be installed in such plants and housed in an effective and economical manner.
In gas separation, individual modules or elements are connected end-to-end, typically in a line of up to about six modules, within a single tube. The tube serves not only to house and support the membrane elements and provide a directed gas flow, but also as the pressure-withstanding vessel that permits operation of the membrane unit at pressures substantially different from the outside atmospheric pressure. The tubes are usually made of stainless or carbon steel, and pressure code-stamped for their intended application.
Various other arrangements have been proposed for arranging multiple membrane modules or elements within a single pressure housing, mostly for reverse osmosis applications.
U.S. Pat. No. 3,774,771 describes an assembly of modules mounted in parallel within a single housing. The modules consist of product tubes around which a piece of reverse osmosis membrane is wound in a helical pattern. Each of these modules is contained within a feed flow tube. The feed flow tubes are connected so that feed can be introduced through an inlet at one end of the housing to several tubes in parallel, can pass down this set of tubes to the other end of the housing, and then pass back along the housing through another set of tubes. Residue is withdrawn from this set of tubes through an outlet adjacent to the feed inlet. Liquid that has permeated the membranes and entered the product tubes is collected and withdrawn through an outlet at the other end of the module.
Each feed flow tube has a hole that allows a volume of feed liquid to leak out from the processing path within the tubes to the space within the housing outside the feed tubes. In this way, pressure is equalized on both sides of the tubes. The assembly is used for reverse osmosis.
U.S. Pat. No. 4,083,780 describes an assembly containing multiple tubes arranged in parallel, with multiple spiral-wound modules in series within each tube.
U.S. Pat. No. 5,238,563 describes an assembly in which multiple membrane modules or elements are housed in parallel. The feed is introduced through a nozzle in the longitudinal shell of the housing and occupies the space between the external surfaces of the modules and the internal surface of the housing.
There remains a need for assemblies that enable gas separation membrane modules to be housed in compact housings that are simple and safe, as well as inexpensive to manufacture, and that provide for easy replacement of modules within the housing. As new gas separation applications develop, some with very large membrane area requirements, this need is more pressing.